Photons have momentum. But is it not misleading to say that they have mass? You also say that E and m are just different words for the same property. This, I believe, is not fair. E (J) differs from m (kg) by a conversion factor of c (m/s) ^2. To give an analogy, I think it would be misleading to say that an electric field E(r) is a different word for some charge Q, because without one you cannot have the other, and they only differ by some conversion factor. Could you explain a little bit more thoroughly what you mean?